This invention relates generally to information recording media, and, more particularly, to recording media of the type in which information can be both recorded and read using optical techniques.
Recording media of this particular type are especially adapted for use both as master discs for subsequent use in the production of replica discs, and for single recording discs that can be directly read while writing so that proper recording can be instantly verified. In both cases, an information signal is typically recorded in such a disc by a thermal process in which an intensity-modulated write beam of light is focused onto the disc as the disc is rotated in a prescribed fashion, thereby changing some characteristic of the disc, such as its reflectivity, in accordance with the information signal.
Recording media that can be used as master discs have typically included a glass substrate having a polished upper surface, with a thin coating of either a photoresist material or a metallic material deposited thereon. In the photoresist embodiment, the intensity-modulated write beam of light functions to expose a sequence of spaced, microscopic areas on the coating, indicative of the information signal being recorded, and a subsequent development of the photoresist material yields a master disc having a sequence of spaced, microscopic pits formed therein. This master disc can then be suitably coated with a thin layer of a metallic material, such as nickel, using an initial step of vacuum deposition by thermal evaporation, followed by a step of electroplating. The metallic layer can then be removed from the master disc to serve as a stamper, with the recorded information signal carried on its underside, for use in a suitable molding apparatus to produce disc replicas.
Although generally satisfactory in yielding master discs that can be used in the production of disc replicas, such photoresist recording media have not proven entirely satisfactory for a number of reasons. One reason for this dissatisfaction arises from an inability to read the recorded disc prior to the development step. This precludes a continuous adjustment of the recording apparatus, to optimize various recording parameters such as beam intensity and focus, as the disc is being recorded. Another drawback arises from a difficulty in producing pits in the medium having a diameter of less than about 0.5 microns, and from the fact that the pits in the disc can sometimes have edges that are ragged and grainy, yielding stampers that can damage the replica discs. Still another drawback arises from the need for an additional step of developing the photoresist material.
In the master disc embodiment that includes a thin metallic film, the intensity-modulated write beam of light functions to melt a sequence of spaced holes in the film, indicative of the information signal. Such a master disc is disclosed in a copending and commonly assigned application for U.S. patent, Ser. No. 890,407, filed in the name of John S. Winslow and entitled "Mastering Machine", now U.S. Pat. No. 4,225,873. Although a signal recorded on such a master disc can be read immediately after being recorded, the relatively high thermal conductivity of the metallic film results in the formation of information-bearing holes in the film that are larger even than those formed in the master discs having photoresist coatings. This limits the recording density that can be achieved. Additionally, a metallic residue is generally formed around each hole, thereby limiting the signal-to-noise ratio that can be achieved during playback of the resultant replica discs.
Recording media that can be used as single recording discs in direct-read-after-write recording apparatus have typically included thin metallic films overlaying plastic substrates or, alternatively, thin layers of highly light-absorptive dielectric materials overlaying substrates having a highly-reflective upper surface. Such discs normally further include a member spaced above the coated substrate to protect the coating from dust and abrasion. In direct-read-after-write recording discs having metallic films, the intensity-modulated beam of light functions to melt a sequence of spaced holes in the film, representative of the information being recorded. However, such metallic film recording discs are subject to the same drawbacks mentioned above, regarding metallic film master recording discs, and thus have not proven entirely successful. Specifically, this includes a limiting recording density and a limited signal-to-noise ratio.
In typical direct-read-after-write recording discs having a dielectric coating, on the other hand, a sequence of spaced-microscopic pits is formed in the dielectric coating by an evaporation or ablating of the coating. Although the pits can be made somewhat smaller in this medium than in the aforementioned metallic film medium, thereby permitting higher recording densities to be realized, the ablating process nevertheless produces a residue of material surrounding each pit that can adversely affect the signal-to-noise ratio that can be achieved during playback.
Thus, it will be appreciated that there is a need for a master recording disc in which even smaller information-bearing surface irregularities can be produced, without the need for a developing step, and without the formation of a residue material or rough edges that can adversely affect both the ultimate formation of replica discs and the signal-to-noise ratio that can be achieved during playback of such replica discs. Similarly, it will be appreciated that there is a need for a direct-read-after-write recording disc having even smaller and residue-free information-bearing surface irregularities, so that higher recording densities and higher signal-to-noise ratios can be achieved. These and other needs are fulfilled by the present invention.